Sys.StringBuilder.prototype.appendFormat = Sys$StringBuilder$appendFormat

function Sys$StringBuilder$appendFormat(mask) {
    this.append(String.format.apply(null, arguments));
}

function TestSB() {
    var sb = new Sys.StringBuilder();
    
    var mask = "{0} goes in front of {1}";
    
    sb.appendFormat(mask, "This", "that");
    
    alert(sb.toString(' - '));
}

